12 Philips Stirling Engines

Work on Stirling engines has been in progress since 1937 al the Research Laboratories of N. V. Philips Gloeilampenfabricken, Eindhoven, Netherlands, the large international company well known for electrical and electronic products.

The work on Stirling engines, extending over 40 years, can be broken into distinct phases:

The initial phase, from 1937 to 1954, was primarily concerned with the establishment oí new ideas and concepts about air engines and the development of small engines.

The cryogenic phase, from 1945 to the present (1978) was concerned with the development and production of Stirling engines working as cryogenic cooling engines.

The r/nmi/?r'c: phase, extending from 1954 to 1970 saw the invention, development, and maturation of single-acting displacer machines with the rhombic-drive mechanism.

The double-acting phase, extending from 1969 to the present (1978) saw emphasis of effort on double-acting engines with the swashplate drive.

ini hal phase

Origin

Interest in Stirling engines at Philips sprang from the need to provide a simple lightweight electric-power generator for their radio receivers and transmitters in areas with no electric-power supply. Various thermally-activated systems were considered, including steam engines and thermoelectric generators. Stirling engines were chosen for development because the actual thermal efficiency of the hot-air engines available or known about at the time was so very low compared with the ideal value. Professor Hoist, director of the program, thought there was a high potential for improvement in Stirling engines with the applicalion of heat-resistant steels and recent knowledge about heat transfer and fluid flow.

II is said that interest in the hot-air engine was stimulated by a visit to the Museum of Technology in Paris by one of the Philips engineers, where he saw hot-air engines exhibited. The early history of the program has not been extensively recorded, however, and this may be apocryphal. It is known that work continued during the German occupation of Eindhoven in the Second World War. but progress was hampered by the need (or secrecy and the lack of heat resisting steels and other materials.

Early publications

The first public announcement of the program» essentially the renaissance of the Stirling engine, was two publications which came out in 1946 (Rinia 1946a), Rinia and tiu Pre 1946). These were followed a year later by two other papers (de Brcy el a I. 1947, and van Weenan 1947). These four papers were in fact the only source publications on the initial phase of the work, but together they represent a substantial foundation for all that was to follow. They excited attention world-wide at the time of their publication and even today are essential reading for those with a serious interest in the subject.

The theory presented by Rinia and du Pre (1946) followed that developed much earlici by Schmidt (1861). In the initial paper mention was made of engines capable of operating at 300(1 revolutions per minute with 'most satisfactory figures for weight and efficiency'! Most of the discussion was about 'air engines', but, in closing, mention was made of the use of Stirling engines as refrigerating machines with alternative working fluids. One experimental engine with hydrogen as the working fluid had attained a temperature of SO K (144 R).

De Brey et al. (1947) discussed considerations for the development of high-speed, high-output engines in terms of the variable parameters in the engine, including the cycle pressure, speed, arid design of heater, cooler, regenerator, and air prehcater. Van Weenan (1947) discussed some aspects of construction of Stilling engines including single-acting two-piston and piston-displacer engines and double-acting machines. Photographs were included of an experimental single-cylinder piston-displacer engine, of about 0.7 kW (I hp) at "2000 revolutions per minute (reproduced in I;ig. 12.11 and the four-cylinder double-acting engine, shown in Fig. 12.2 delivering 11 kW (15 hp) al 3000 revolutions per minute. Further papers were promised containing details of the test results on these engines, but unfortunately these were never published. In his paper, van Weenan mentions that double-acting engines with a wobble-plate drive were thought to be limited to 15 to 22 kW (20 to 30 hp) capacity. For higher powers other driving mechanisms had been worked out based on an arrangement of "the cylinders in V form one behind the other". An early engine of this configuration is shown in Tig. 12.3.

Double-acting engines

I'he existence of double-acting engines at this early stage in the Philips work is remarkable. The principle was described by Rinia (1946a) but the

first discussion of Substance was given by van Weenan (1947) along with the photograph of the high speed 11 kW (15 hp) engine. Even more remarkable, however, is the historical fact that double-acting engines were invented nearly a century before. Babcock (1885) ascribes the original double-acting engine to Charles I .ouis Franchol, who patented

the double-acting two-cylinder configuration in 1853. Babcock (1885) also describes the invention, patented in I860 by Sir William Siemens, of an engine having Tom cylinders each, hot at one end and cold at the other, ail connected to one shaft through a wobbling disc at equal quarters of the revolution and so arranged that the hot end of one communicated through an economizer with the cold end of the other in order'.

1'his describes exactly the multiple-cylinder double-acting Philips engines of the 1916 era and also I he present-day machines developed following a return to the double-acting engine in the 1970s. So far as is known Sir William did not reduce his novel concept to practice. He was prolific with ideas and concepts but his perception was truly remarkable. Babcock in his account provides a prophetic comment on the promise of the Siemens engine foi development. It is not known il Siemens' engine was known to Philips or if the engine was reinvented 80 years after the original.

Small power generators It is difficult to overestimate the accomplishment of the Philips engineers in this initial phase of the program. As Ross (1977) says:

'In the few short years from their lir«;t tentative interest in 1937 until these articles in 1947 and with a World War in between Philips had taken the Stirling engine and increased its power per pound by n factor of fifty, reduced its size per unit of power by a factor of 125 and increased its speed by a factor of ten.'

Work on the small fractional power engine generators continued in the late 1940s. Market surveys indicated the need for a self-contained electric-power generating set of 150 to 200 waits (0.2 to 0.27 hp). Eventually, after much detailed engineering work, lite Type 102C shown in Fig. 12.4 evolved. This unit was to receive considerable development including a duration test of 21100 hours without maintenance, high-altitude testing on the Jungfrau .loch in the Swiss Alps, in thin air at temperatures of 23 ( 9.4 °F). Low temperature tests lo -40°C(-40°F) were also carried out. Ross (1977) describes one lest in which an engine with water cooler and other modifications to increase the power to 2 kW (2.7 hp) was fitted to a row boat and motored for 80.5 km (50

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.